To contain cost and reduce complexity, it is a goal of analog circuit design to implement analog functions that have traditionally been performed by discrete components on integrated circuit chips. An analog signal proesssing function found in many analog systems is a low-pass filter. Low-pass filters receive an input signal having a range of frequency components, filter out high frequencies components from the signal, and allow only low frequency components of the input signal to propagate.
Two examples are of low-pass filter implementations of the prior art are shown in FIGS. 1 and 2. FIG. 1 illustrates a continuous time, low-pass filter implemented in an integrator configuration. An input signal enters the filter at Vin, is integrated by a resistor, capacitor, and operational amplifier, and an integrated output signal exits at Vout. Values are chosen for R and C in FIG. 1 to set a low frequency pole (.function..sub.0) which determines the frequency response of the low-pass filter using the following equation: ##EQU1##
Attempts have been made to implement the circuit of FIG. 1 on an integrated circuit ("IC") chip. Implementation is a problem, however, because a resistance (R) of about 10 K.OMEGA. is the maximum value practically achievable using passive resistors on an IC chip. This maximum is not absolute, but is dictated by the processing techniques used and the chip area required to make a resistor. Because 10 K.OMEGA. is a relatively small value, in order to set a low frequency pole at an ideal range for low pass filtering of between 1 and 10 Khz, the value of capacitor must be between approximately 1.5 and 15 nF. These values of capacitance are much too large to be practically implemented on an IC chip. Therefore, off-chip capacitors must be used to produce the necessary capacitance for C in FIG. 1, resulting in a forfeiture of at least two input/output ("I/O") pins on the IC chip which must be connected to the terminals of the capacitor. In addition, the I/O pins and off-chip wiring required to connect an external capacitor to the filter circuit of FIG. 1 act as antennas that couple a substantial quantity of high-frequency card or module noise back into the filter circuit thus reducing the effectiveness of the filter circuit.
FIG. 2 shows a switched capacitor implementation of a low-pass filter. In this configuration, a continuous time resistor is simulated by the clocked switches surrounding the capacitor C1. The clock phases .PHI..sub.1 and .PHI..sub.2 are generated at a specified frequency and are typically non-overlapping. The frequency of the clock and the values of C 1 and C2 are selected for a desired low frequency pole determined by the following equation where T is the period of the clock: ##EQU2##
The switched capacitor technique has a number of disadvantages when implemented on an integrated circuit. First, the clocks are generated external to the chip and must be brought onto the chip. Therefore, additional I/O pins are required. Also, the clock signal lines themselves couple high-frequency noise from the card or module into the filter circuitry thus reducing the effectiveness of the low-pass filter. Second, in order for the switched circuit capacitor circuit to function properly, the frequency of the clock signals must be nuch greater than the high frequency components of the input signal. This may not practical depending on the frequency of interest of the input signal. Finally, additional low-pass filter circuitry is required in switched capacitor circuits for aliasing.
There is a need for a high valued active device resistor for use in a low bandpass filter and other analog signal processing applications so that these analog signal processing applications can be implemented entirely on an IC chip.